The present invention relates to a process for manufacturing, by means of electrical discharge machining (EDM) techniques, an impeller or a rotor having one or more impellers, in a turbine pump, particularly a turbomolecular pump, as well as to the impeller or the rotor obtained through such process.
A rotor in a turbomolecular pump is made up by a series of impellers that are keyed to a rotating shaft. Each impeller comprises a disk equipped with an array of radial peripheral blades which rotate, in use, at a speed of up to several tens of thousands rpm. The severe conditions of use and the ever increasing performance demands for turbomolecular pumps as to the compression and the pumping speed, require that the structure of each single impeller and of the rotor as a whole, be sturdy and balanced, and that the shape of the impeller blades is such as to optimize the pump performance.
The known processes for manufacturing impeller blades of turbomolecular pumps require the separate manufacture of the impellers and then their keying to the rotor shaft.
According to British Patent application No. 2 171 942, each impeller is built by milling the blades thereof by means of an apparatus that allows for the simultaneous machining of several disks. Nevertheless such machining requires several steps and is such as to impose limitations to the article characteristics: for example the gap between the blades cannot be smaller than certain values determined by the cutting methods employed. Moreover, the problems relating to keying the impellers to the shaft remain unsolved.
Another known approach is that of forming the single blades of the impeller by means of the electro-discharge machining technique. The use of this technique improves some characteristics of the article, e.g., reducing the gap between the blades when compared with that of a milling process, but nevertheless the manufacture process is still a long and costly one since the blades are built one by one and the single impellers have to be keyed to the rotor shaft. An example of this manufacture technique, which is not applied to rotors for turbomolecular pumps, is described in French Patent application No. 2 670 970.
DE-A-27 17 366 discloses an impeller with inclined blades for a turbomolecular pump, wherein the blade width, their thickness and their number are related by the following relationship: EQU (a/tan .alpha.)+(d/sin .alpha.)=2.pi.r/z
where .alpha. is the angle between the tangent to the radius r of each blade and the rotor plane. This document does not describe the process for manufacturing the impeller, nor does its disclose an impeller which is integral with the shaft.
An object of the present invention is that of overcoming or reducing the drawbacks of the known processes for manufacturing rotors of turbomolecular pumps, by providing a process that is both quick and adapted to produce an impeller or a rotor having high operating characteristics.